It is known that concrete can be restored by generating movement of ions within the concrete structure between an anode located on the exterior of the concrete structure and a cathode defined by the conventional metal reinforcing members within the concrete. An electrolyte is carried in a porous material between the outside surface of the concrete and the anode.
Various restorative effects can be obtained using this method including particularly the extraction of chloride ions from the concrete which would otherwise cause corrosion of the metal reinforcement leading to degradation of this structure and spalling of the concrete material covering the reinforcing members.
Examples of this method are shown and described in detail in a brochure by Norcure Chloride Removal Systems Inc. entitled "Is Salt Induced Corrosion Causing Problems with your Concrete Structures", in a brochure by Vector Construction entitled "The Concrete Restoration and Protection Specialists" and in a brochure by "Fosroc/NCTAS" entitled "Norcure Desalination". These brochures describe a technique which is used for various concrete structures including bridge decks and the brochure by Fosroc shows particularly a technique in which a bridge deck is restored using this anodic method.
In the brochure and as generally used in practice, after the concrete surface is exposed by removal of any covering layers, a porous material is laid down onto the upper surface and this receives an electrolyte. The porous material is then covered by a mesh type electrode in the form of wire netting which is then covered by a further layer of the porous material.
A current supply is connected between the mesh anode and the reinforcing steel of the concrete and over an extended period of many weeks this acts to cause transfer of ions from the concrete material through the electrolyte to provide a restorative effect.
However the conventional technique using the layers of porous material and the wire mesh anode require that the bridge deck be placed out of operation for the extended period of the restoration. While this can be accommodated in some cases, it does of course severely restrict traffic flow.
The increased usage of salt as a de-icing agent in freezing conditions has severely exacerbated the problem of chloride degradation of concrete.
Up till now, therefore, in practice it has not been possible to operate this technique without the necessity for closing the concrete structure to passing vehicles or traffic and this disadvantage has significantly reduced the use of this restorative technique.
U.S. Pat. No. 5,296,120 (Bennett et al assigned to ELTECH Systems Corporation and issued Mar. 22, 1994) discloses an anode system for use in the above method. However this technique as disclosed in the patent has achieved little or no commercial success and has in effect been abandoned. The main disclosure of the patent relates to an anode construction which can be rolled into a supply roll for transportation and then can be unrolled into place. The anode is provided by a mesh of a suitable metal, preferably a film forming valve metal base which is combined into a structure including one or more layers of cellulose and polymeric foams. The patent also mentions that the foam layer can be made conductive by the addition of a conductive material such as carbon powder. However this is intended to be used with the metal mesh anode simply to enhance conductivity.
A further example is disclosed in U.S. Pat. No. 5,141,607 issued Aug. 25th 1992 (Swiat assigned to Corrpro Companies Inc) which discloses an anode which is of the mesh-type embedded within a porous material.
Restoration of concrete using a temporary current is an entirely different process from impressed current cathodic protection. In the latter process, a small current of the order of 1-10 mAmps/sq meter is caused to flow continuously through the life of the concrete for the purpose of inhibiting corrosion.
The current used in the restoration process is strictly temporary for a period of the order of 20 to 90 days and has a value which of the order of 50 to 200 TIMES that of the continuous current. Thus the current in the restoration process may lie in the range 0.4 to 3.0 Amps/sq meter. In addition, the process of restoration must include a liquid electrolyte whereas the continuous process is dry. Therefore the types of anode and materials to be used are of an entirely different character.
It is however known that a continuous current for cathodic protection to inhibit corrosion can be generated using a conductive asphalt in which a carbon powder has been added to provide the necessary conductivity.